A method was developed to determine rates of incorporation of palmitic acid into different brain regions in the awake rat. The regional cerebral metabolic rate for palmitate, rCMRpalm, was determined. rCMRpalm ranged from 2 x 10-5 mol/g-sec at the internal capsule to 9.3 x 10-5 mol/g-sec at the median eminence, and was proportional to the regional cerebral metabolic rate for glucose. rCMRpalm is a measure of turnover of structural brain lipids in vivo. In Fischer-344 rats at different ages, rCMRpalm declines between 1 and 3 months, when myelination also declines, but is age invariant between 3 and 34 months. The latter findings indicates maintenance of cerebral integrity in the absence of disease. rCMRpalm is independent of cerebral blood flow. It is unaffected by acute visual or auditory deprivation in awake rats, but is reduced by anesthesia induced by pentobarbital.